[Scaevolus]

It's malicious in the sense that infected machine resources are being used to directly turn a profit for the controller.

Other ways of profiting off botnets include renting out DDoS and spam capabilities.

[sliverstorm]

Not just turn a profit for the controller, but incur a cost for the victim. I measured my computer the other day; when I start mining, my power draw jumps 160 watts, and I have a fairly efficient machine.